Judging by the way the wing folded, it will be the combined shear stress and internal bending moment at the inboard wing fittings that need to be looked at. The PA-25 has a strutted wing which could mean a local bending moment of zero at the bolts, leaving the shear stress as the remaining factor. A change in G-loading will influence this and I was just trying to illustrate that dumping a hopper full of chemicals could have contributed significantly to the total stresses in the structure, even though at first 'lightening the load' appears to be a positive thing. I think that's as far as we need to go with the theories...